Download - Modulation Techniques for Mobile Radio
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Modulation Techniques for Mobile RadioModulation Techniques for Mobile RadioModulation Techniques for Mobile RadioModulation Techniques for Mobile Radio
Modulation is the process of encoding the baseband or source information (voice, video, text) in a manner suitable for transmission.
It generally involves translating a base band signal (or source) to a band pass signal, centered at a high carrier frequency.
Demodulation is the process of extracting the base band message from the carrier.
Modulation is the process of encoding the baseband or source information (voice, video, text) in a manner suitable for transmission.
It generally involves translating a base band signal (or source) to a band pass signal, centered at a high carrier frequency.
Demodulation is the process of extracting the base band message from the carrier.
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Modulation TechniquesModulation TechniquesModulation TechniquesModulation Techniques
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Review of Analog Modulation TechniquesReview of Analog Modulation TechniquesReview of Analog Modulation TechniquesReview of Analog Modulation Techniques
Amplitude Modulation (AM)Amplitude Modulation (AM)
• Message Signal -- Message Signal --
• Carrier SignalCarrier Signal -- --
• AM Signal AM Signal ----
Amplitude Modulation (AM)Amplitude Modulation (AM)
• Message Signal -- Message Signal --
• Carrier SignalCarrier Signal -- --
• AM Signal AM Signal ----
c cA cos(2πf t)
AM c cS (t) = A [1+m(t)]cos(2 f t)
m(t)
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AM SpectrumAM SpectrumAM SpectrumAM Spectrum
AMS (f) c c c c c = 0.5A [ (f - f ) + M(f - f ) + (f + f ) + M(f + f )]
CarrierSidebands
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AM ParametersAM ParametersAM ParametersAM Parameters• Modulation IndexModulation Index -- --
• BandwidthBandwidth -- --
• Total Power in AM SignalTotal Power in AM Signal
• Power in the carrierPower in the carrier -- --
• Modulation IndexModulation Index -- --
• BandwidthBandwidth -- --
• Total Power in AM SignalTotal Power in AM Signal
• Power in the carrierPower in the carrier -- --
AM m B = 2 f
AMPc
2 2= 0.5A [ 1 + 2 <m(t)> + <m (t)> ]
c
2c P = A / 2
m c k= A / A 1
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Single Singleband AM SignalSingle Singleband AM SignalSingle Singleband AM SignalSingle Singleband AM SignalLower SidebandLower Sideband
Upper SidebandUpper Sideband
Where the Hilbert transform is defined as:Where the Hilbert transform is defined as:
; ;
Lower SidebandLower Sideband
Upper SidebandUpper Sideband
Where the Hilbert transform is defined as:Where the Hilbert transform is defined as:
; ;
SSBS c c cA [m(t)cos(2 f t) m(t)sin(2 f t)]
m(t) m(t) h(t) h(t) 1/ t
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SSB GenerationSSB GenerationSSB GenerationSSB GenerationFilter MethodFilter Method
Baseband filter passes upper or lower sidebands Baseband filter passes upper or lower sidebands
Filter MethodFilter Method
Baseband filter passes upper or lower sidebands Baseband filter passes upper or lower sidebands
Baseband FilterBaseband Filter
SSBS (t)c cA cos(2 f t)m(t)
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Balanced ModulatorBalanced ModulatorBalanced ModulatorBalanced Modulator
SSBS (t)Carrier fc
-90-90o o phase shiftphase shift
9090oo
∑∑
m(t)
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Properties of SSBProperties of SSBProperties of SSBProperties of SSB
Bandwidth of SSB is very efficient = fBandwidth of SSB is very efficient = fmm .However, Doppler spreading and Rayleigh .However, Doppler spreading and Rayleigh
fading can shift the signal spectrum, causing fading can shift the signal spectrum, causing distortion.distortion.
Frequency of the receiver oscillator must be Frequency of the receiver oscillator must be exactly the same as that of the transmitted exactly the same as that of the transmitted carrier fcarrier fcc. If not, this results in a frequency . If not, this results in a frequency shift fshift fcc f, causing distortion.f, causing distortion.
Bandwidth of SSB is very efficient = fBandwidth of SSB is very efficient = fmm .However, Doppler spreading and Rayleigh .However, Doppler spreading and Rayleigh
fading can shift the signal spectrum, causing fading can shift the signal spectrum, causing distortion.distortion.
Frequency of the receiver oscillator must be Frequency of the receiver oscillator must be exactly the same as that of the transmitted exactly the same as that of the transmitted carrier fcarrier fcc. If not, this results in a frequency . If not, this results in a frequency shift fshift fcc f, causing distortion.f, causing distortion.
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Pilot Tone SSB Pilot Tone SSB Pilot Tone SSB Pilot Tone SSB
Transmit a low level pilot tone along with Transmit a low level pilot tone along with the SSB signal.the SSB signal.
The pilot tone has information on the frequency The pilot tone has information on the frequency and amplitude of the carrier.and amplitude of the carrier.
The pilot tone can be tracked using signal The pilot tone can be tracked using signal processing FFSR - Feed Forward Signal processing FFSR - Feed Forward Signal Regeneration.Regeneration.
Transmit a low level pilot tone along with Transmit a low level pilot tone along with the SSB signal.the SSB signal.
The pilot tone has information on the frequency The pilot tone has information on the frequency and amplitude of the carrier.and amplitude of the carrier.
The pilot tone can be tracked using signal The pilot tone can be tracked using signal processing FFSR - Feed Forward Signal processing FFSR - Feed Forward Signal Regeneration.Regeneration.
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TTIB (Transparent Tone In-Band) SystemTTIB (Transparent Tone In-Band) System TTIB (Transparent Tone In-Band) SystemTTIB (Transparent Tone In-Band) System
~
m(t)
a
a
b
c d
2f1f f~ ~
~ e
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a
d
bc
e
frequencyf2 1BW f f
1f
2f
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Properties of TTIB systemProperties of TTIB systemProperties of TTIB systemProperties of TTIB system
Base band signal is split into two equal width Base band signal is split into two equal width segments.segments.
Small portion of audio spectrum is removed and Small portion of audio spectrum is removed and a low-level pilot tone is inserted in its place.a low-level pilot tone is inserted in its place.
This procedure maintains the low bandwidth of This procedure maintains the low bandwidth of the SSB signal.the SSB signal.
Provides good adjacent channel protection.Provides good adjacent channel protection.
Base band signal is split into two equal width Base band signal is split into two equal width segments.segments.
Small portion of audio spectrum is removed and Small portion of audio spectrum is removed and a low-level pilot tone is inserted in its place.a low-level pilot tone is inserted in its place.
This procedure maintains the low bandwidth of This procedure maintains the low bandwidth of the SSB signal.the SSB signal.
Provides good adjacent channel protection.Provides good adjacent channel protection.
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Demodulation of AM signalsDemodulation of AM signals Demodulation of AM signalsDemodulation of AM signals
• Coherent Modulation
• Non-coherent demodulation
• Envelope Detectors
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Demodulation of AM signalsDemodulation of AM signals
Coherent ModulationCoherent Modulation
Demodulation of AM signalsDemodulation of AM signals
Coherent ModulationCoherent Modulation
LPFLPF o c oA cos(2 f t ) o c oA cos(2 f t )
OUT o r oV (t) 0.5A R(t)cos( ) OUT o r oV (t) 0.5A R(t)cos( )
AM c rS R(t)cos(2 f t )
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Frequency ModulationFrequency ModulationFrequency ModulationFrequency Modulation• Message Signal –Message Signal –
• FM Signal –FM Signal –
• Power in FM Signal –Power in FM Signal –
• Frequency modulationFrequency modulation index index ––
W = Highest frequency component in message signalW = Highest frequency component in message signalAAM M = Peak value of modulating signal= Peak value of modulating signal
• Message Signal –Message Signal –
• FM Signal –FM Signal –
• Power in FM Signal –Power in FM Signal –
• Frequency modulationFrequency modulation index index ––
W = Highest frequency component in message signalW = Highest frequency component in message signalAAM M = Peak value of modulating signal= Peak value of modulating signal
m(t)
FM C c f
t
S (t) A cos[2 f t 2 k m(t)dt]
f f Mk A / W f / W
FM c
2P A / 2
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Phase ModulationPhase ModulationPhase ModulationPhase Modulation
• PM Signal PM Signal
• Phase Modulation Index Phase Modulation Index
• Power in PM signal Power in PM signal
• Bandwidth Bandwidth
• PM Signal PM Signal
• Phase Modulation Index Phase Modulation Index
• Power in PM signal Power in PM signal
• Bandwidth Bandwidth
PM c
2P A / 2
Mk A
TB 2 f
SPM (t) = Ac cos[2fc t +km(t)]SPM (t) = Ac cos[2fc t +km(t)]
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FM methodsFM methods FM methodsFM methods
FM Modulation FM Modulation Direct Method – VCODirect Method – VCO Indirect Method – ArmstrongIndirect Method – ArmstrongFM DetectionFM Detection Slope DetectionSlope Detection Zero Crossing DetectionZero Crossing Detection PLL DetectionPLL Detection Quadrature DetectionQuadrature Detection
FM Modulation FM Modulation Direct Method – VCODirect Method – VCO Indirect Method – ArmstrongIndirect Method – ArmstrongFM DetectionFM Detection Slope DetectionSlope Detection Zero Crossing DetectionZero Crossing Detection PLL DetectionPLL Detection Quadrature DetectionQuadrature Detection
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Comparison between AM and FM Comparison between AM and FM Comparison between AM and FM Comparison between AM and FM
FMFM AMAM•FM signals are less noisy, FM signals are less noisy, because amplitude of signal because amplitude of signal is constantis constant
•AM signals are more noisy, AM signals are more noisy, amplitude cannot be limitedamplitude cannot be limited
•The modulation index can The modulation index can be varied to obtain greater be varied to obtain greater SNR(6dB for each doubling SNR(6dB for each doubling in bandwidth)in bandwidth)
•Modulation index cannot be Modulation index cannot be changed automatically.changed automatically.
•FM signals occupy more FM signals occupy more bandwidth (good for audio)bandwidth (good for audio)
•AM signals occupy lesser AM signals occupy lesser bandwidth (good for video)bandwidth (good for video)
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Digital ModulationDigital ModulationDigital ModulationDigital Modulation VLSI and DSP promoted the advent of Digital VLSI and DSP promoted the advent of Digital
ModulationModulation Low noiseLow noise Easier multiplexing of information (voice, data, Easier multiplexing of information (voice, data,
video)video) Can accommodate digital transmission errors, Can accommodate digital transmission errors,
source coding, encryption and equalization.source coding, encryption and equalization. DSP can implement digital modulators, DSP can implement digital modulators,
demodulators completely in software.demodulators completely in software.
VLSI and DSP promoted the advent of Digital VLSI and DSP promoted the advent of Digital ModulationModulation
Low noiseLow noise Easier multiplexing of information (voice, data, Easier multiplexing of information (voice, data,
video)video) Can accommodate digital transmission errors, Can accommodate digital transmission errors,
source coding, encryption and equalization.source coding, encryption and equalization. DSP can implement digital modulators, DSP can implement digital modulators,
demodulators completely in software.demodulators completely in software.
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Basics of digital communicationsBasics of digital communications Basics of digital communicationsBasics of digital communications
In digital communication systems, the In digital communication systems, the message) is represented as a time sequence of message) is represented as a time sequence of symbols or pulses. symbols or pulses.
Each symbol has m finite statesEach symbol has m finite states
Number of bits required for m states:Number of bits required for m states:n = logn = log22m bits/symbol m bits/symbol
In digital communication systems, the In digital communication systems, the message) is represented as a time sequence of message) is represented as a time sequence of symbols or pulses. symbols or pulses.
Each symbol has m finite statesEach symbol has m finite states
Number of bits required for m states:Number of bits required for m states:n = logn = log22m bits/symbol m bits/symbol
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Shannon’s bandwidth theorem Shannon’s bandwidth theorem Shannon’s bandwidth theorem Shannon’s bandwidth theorem
Bandwidth efficiencyBandwidth efficiency BB = R / B bps/Hz = R / B bps/Hz
R=Data rate in bits/secondR=Data rate in bits/second
B=Bandwidth of modulated RF signalB=Bandwidth of modulated RF signal Shannon's formula:Shannon's formula:
BmaxBmax = C/B = = C/B = channel capacity (bits/s)channel capacity (bits/s) RF bandwidthRF bandwidth
= log= log22(1 + S/N)(1 + S/N)S/N = Signal to Noise ratioS/N = Signal to Noise ratio
Bandwidth efficiencyBandwidth efficiency BB = R / B bps/Hz = R / B bps/Hz
R=Data rate in bits/secondR=Data rate in bits/second
B=Bandwidth of modulated RF signalB=Bandwidth of modulated RF signal Shannon's formula:Shannon's formula:
BmaxBmax = C/B = = C/B = channel capacity (bits/s)channel capacity (bits/s) RF bandwidthRF bandwidth
= log= log22(1 + S/N)(1 + S/N)S/N = Signal to Noise ratioS/N = Signal to Noise ratio
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Practical digital systemsPractical digital systems For US digital cellular standard, R = 48.6 kbpsFor US digital cellular standard, R = 48.6 kbps
RF bandwidth = 30 KHzRF bandwidth = 30 KHz
For SNR 20 dB => 100For SNR 20 dB => 100
C = 30000 * logC = 30000 * log22(1 + S/N)(1 + S/N)
= 30000 * log= 30000 * log22(1 + 100) = 199.75 kbps(1 + 100) = 199.75 kbps For GSM standard, R = 270.833 kbpsFor GSM standard, R = 270.833 kbps
C = 1.99 Mbps for S/N = 30 dBC = 1.99 Mbps for S/N = 30 dB
Practical digital systemsPractical digital systems For US digital cellular standard, R = 48.6 kbpsFor US digital cellular standard, R = 48.6 kbps
RF bandwidth = 30 KHzRF bandwidth = 30 KHz
For SNR 20 dB => 100For SNR 20 dB => 100
C = 30000 * logC = 30000 * log22(1 + S/N)(1 + S/N)
= 30000 * log= 30000 * log22(1 + 100) = 199.75 kbps(1 + 100) = 199.75 kbps For GSM standard, R = 270.833 kbpsFor GSM standard, R = 270.833 kbps
C = 1.99 Mbps for S/N = 30 dBC = 1.99 Mbps for S/N = 30 dB
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Line CodingLine CodingLine CodingLine Coding Line codes are used to provide particular spectral Line codes are used to provide particular spectral
characteristics of a pulse train.characteristics of a pulse train. Line codes provide the pulses to represent 0s Line codes provide the pulses to represent 0s
and 1s.and 1s. Line codes can be:Line codes can be:
o Return-to-zero (RZ)Return-to-zero (RZ)o Non-return-to-zero (NRZ)Non-return-to-zero (NRZ)
Line codes are Unipolar (0,V) or Bipolar (-V, V )Line codes are Unipolar (0,V) or Bipolar (-V, V )
Line codes are used to provide particular spectral Line codes are used to provide particular spectral characteristics of a pulse train.characteristics of a pulse train.
Line codes provide the pulses to represent 0s Line codes provide the pulses to represent 0s and 1s.and 1s.
Line codes can be:Line codes can be:o Return-to-zero (RZ)Return-to-zero (RZ)o Non-return-to-zero (NRZ)Non-return-to-zero (NRZ)
Line codes are Unipolar (0,V) or Bipolar (-V, V )Line codes are Unipolar (0,V) or Bipolar (-V, V )
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Unipolar NRZUnipolar NRZ 1 0 1 1 0 1 0 1 1 0
Unipolar NRZUnipolar NRZ 1 0 1 1 0 1 0 1 1 0
VV
0 Unipolar RZ0 Unipolar RZ
VV Bipolar NRZ Bipolar NRZ
-V-V
VV
0 Unipolar RZ0 Unipolar RZ
VV Bipolar NRZ Bipolar NRZ
-V-V
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Pulse Shaping TechniquesPulse Shaping TechniquesPulse Shaping TechniquesPulse Shaping Techniques
ISI – Inter Symbol Interference ISI – Inter Symbol Interference errors in errors in transmission of symbolstransmission of symbols
Pulse shaping techniques reduce the inter-Pulse shaping techniques reduce the inter-symbol effectssymbol effects
ISI – Inter Symbol Interference ISI – Inter Symbol Interference errors in errors in transmission of symbolstransmission of symbols
Pulse shaping techniques reduce the inter-Pulse shaping techniques reduce the inter-symbol effectssymbol effects
BandlimitedBandlimitedChannelChannel
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Pulse shaping filtersPulse shaping filtersPulse shaping filtersPulse shaping filters Raised cosine filterRaised cosine filter
As the value of As the value of (roll-off factor) increases, the (roll-off factor) increases, the bandwidth of the filter also increasesbandwidth of the filter also increases
As the value of a (roll-off factor) increases, the As the value of a (roll-off factor) increases, the time sidelobe levels decrease. time sidelobe levels decrease.
Raised cosine filterRaised cosine filter
As the value of As the value of (roll-off factor) increases, the (roll-off factor) increases, the bandwidth of the filter also increasesbandwidth of the filter also increases
As the value of a (roll-off factor) increases, the As the value of a (roll-off factor) increases, the time sidelobe levels decrease. time sidelobe levels decrease.
RCh (t)
s2
s
cos( t /T )sin( t /T ) s[ ]( t /T ) (1 4 t /2T )s
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Implementation of raised-cosine filterImplementation of raised-cosine filter Implementation of raised-cosine filterImplementation of raised-cosine filter
Use identical [HUse identical [HRCRC (f)] (f)]1/21/2 filters at transmitter and filters at transmitter and receiver receiver
Symbol rate possible through raised cosine filter Symbol rate possible through raised cosine filter
where B is the filter bandwidthwhere B is the filter bandwidth
Use identical [HUse identical [HRCRC (f)] (f)]1/21/2 filters at transmitter and filters at transmitter and receiver receiver
Symbol rate possible through raised cosine filter Symbol rate possible through raised cosine filter
where B is the filter bandwidthwhere B is the filter bandwidth
s sR 1/ T 2B /(1 )
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Types of Digital ModulationTypes of Digital ModulationTypes of Digital ModulationTypes of Digital ModulationLinearLinear Non-LinearNon-Linear Spread Spread
SpectrumSpectrumAmplitude of Amplitude of transmitted signal transmitted signal varies linearly with varies linearly with message signal m(t) message signal m(t)
Amplitude of Amplitude of carrier is constant carrier is constant
Transmission Transmission bandwidth >> bandwidth >> signal signal bandwidth bandwidth
Low bandwidth- Low bandwidth- allows more usersallows more users
High bandwidth –High bandwidth –Low noiseLow noise
More users-More users-high bandwidthhigh bandwidth
Example systems:Example systems: BPSK, QPSKBPSK, QPSK
FSK, GMSKFSK, GMSK W-CDMA, W-CDMA, cdma 2000cdma 2000
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Linear digital modulationLinear digital modulationLinear digital modulationLinear digital modulation
PSK or Phase Shift Keying of carrier:PSK or Phase Shift Keying of carrier:SSPSKPSK = A cos( = A cos(t + t + kk))
kk = 0, = 0, (BPSK) (BPSK) kk = 0, = 0, (QPSK) (QPSK) kk = =
0,0,330PSK)0PSK)
PSK or Phase Shift Keying of carrier:PSK or Phase Shift Keying of carrier:SSPSKPSK = A cos( = A cos(t + t + kk))
kk = 0, = 0, (BPSK) (BPSK) kk = 0, = 0, (QPSK) (QPSK) kk = =
0,0,330PSK)0PSK)
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Constellation DiagramConstellation DiagramConstellation DiagramConstellation Diagram
Q(Quadrature)Q(Quadrature)
I (In Phase)I (In Phase)
Q(Quadrature)Q(Quadrature)
I (In Phase)I (In Phase)
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Properties of BPSK and QPSKProperties of BPSK and QPSKProperties of BPSK and QPSKProperties of BPSK and QPSK BPSKBPSK
BW = 2 RBW = 2 RBB = 2 / T = 2 / TBB
PPe,BPSKe,BPSK = Q[√(2 E = Q[√(2 EBB / N / N00)])]
RRBB – Bit rate, T – Bit rate, TBB – Bit period – Bit period
EEBB – Energy/bit, N – Energy/bit, N00 – Noise spectral density – Noise spectral density QPSKQPSK
BW = RBW = RBB = 1 / T = 1 / TBB
PPe,QPSKe,QPSK = Q[√(2 E = Q[√(2 EBB / N / N00)])]
BPSKBPSK
BW = 2 RBW = 2 RBB = 2 / T = 2 / TBB
PPe,BPSKe,BPSK = Q[√(2 E = Q[√(2 EBB / N / N00)])]
RRBB – Bit rate, T – Bit rate, TBB – Bit period – Bit period
EEBB – Energy/bit, N – Energy/bit, N00 – Noise spectral density – Noise spectral density QPSKQPSK
BW = RBW = RBB = 1 / T = 1 / TBB
PPe,QPSKe,QPSK = Q[√(2 E = Q[√(2 EBB / N / N00)])]
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Nonlinear or envelope modulationNonlinear or envelope modulationNonlinear or envelope modulationNonlinear or envelope modulation
Frequency shift keying Frequency shift keying
The frequency of a constant amplitude carrier The frequency of a constant amplitude carrier signal is switched between 2 values ( 1 and 0)signal is switched between 2 values ( 1 and 0)
Frequency shift keying Frequency shift keying
The frequency of a constant amplitude carrier The frequency of a constant amplitude carrier signal is switched between 2 values ( 1 and 0)signal is switched between 2 values ( 1 and 0)
1/ 2FSK h b b c bS V (t) (2E / T ) cos[2 f 2 f ]t,0 T T
1/ 2FSK i b b c bS V (t) (2E / T ) cos[2 f 2 f ]t,0 T T
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Properties of FSK Properties of FSK Properties of FSK Properties of FSK Transmission BandwidthTransmission Bandwidth
BBTT = 2 = 2f + 2Bf + 2B
B = Bandwidth of digital base-band signalB = Bandwidth of digital base-band signal If a raised cosine pulse-shaping filter is usedIf a raised cosine pulse-shaping filter is used
BBTT = 2 = 2f + (1 + f + (1 + )R)R Probability of error Probability of error
PPe,FSKe,FSK = Q[(E = Q[(EBB / N / N00))1/21/2]]
Transmission BandwidthTransmission Bandwidth
BBTT = 2 = 2f + 2Bf + 2B
B = Bandwidth of digital base-band signalB = Bandwidth of digital base-band signal If a raised cosine pulse-shaping filter is usedIf a raised cosine pulse-shaping filter is used
BBTT = 2 = 2f + (1 + f + (1 + )R)R Probability of error Probability of error
PPe,FSKe,FSK = Q[(E = Q[(EBB / N / N00))1/21/2]]
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Spread Spectrum Modulation techniquesSpread Spectrum Modulation techniques Spread Spectrum Modulation techniquesSpread Spectrum Modulation techniques
Spread spectrum techniques employ a Spread spectrum techniques employ a transmission bandwidth >> signal bandwidthtransmission bandwidth >> signal bandwidth
The system is inefficient for a single user, but The system is inefficient for a single user, but is efficient for many usersis efficient for many users
Many users use the same bandwidth without Many users use the same bandwidth without significantly interfering with one anothersignificantly interfering with one another
Spread spectrum techniques employ a Spread spectrum techniques employ a transmission bandwidth >> signal bandwidthtransmission bandwidth >> signal bandwidth
The system is inefficient for a single user, but The system is inefficient for a single user, but is efficient for many usersis efficient for many users
Many users use the same bandwidth without Many users use the same bandwidth without significantly interfering with one anothersignificantly interfering with one another
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Principle of Spread SpectrumPrinciple of Spread SpectrumPrinciple of Spread SpectrumPrinciple of Spread Spectrum
Spread spectrum signals are PN (pseudo – Spread spectrum signals are PN (pseudo – noise) sequence or code.noise) sequence or code.
Spread spectrum signals are demodulated at Spread spectrum signals are demodulated at the receiver by cross correlation with the the receiver by cross correlation with the correct PN sequence.correct PN sequence.
PN codes are approximately orthogonal, and PN codes are approximately orthogonal, and the receiver can separate each user based on the receiver can separate each user based on their codes.their codes.
Spread spectrum signals are PN (pseudo – Spread spectrum signals are PN (pseudo – noise) sequence or code.noise) sequence or code.
Spread spectrum signals are demodulated at Spread spectrum signals are demodulated at the receiver by cross correlation with the the receiver by cross correlation with the correct PN sequence.correct PN sequence.
PN codes are approximately orthogonal, and PN codes are approximately orthogonal, and the receiver can separate each user based on the receiver can separate each user based on their codes.their codes.
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Advantages of spread spectrum techniquesAdvantages of spread spectrum techniquesAdvantages of spread spectrum techniquesAdvantages of spread spectrum techniques
Spread spectrum communications (3G) offer Spread spectrum communications (3G) offer high bandwidth compared to 1G and 2G high bandwidth compared to 1G and 2G systems.systems.
Resistance to multi-path fading, because of Resistance to multi-path fading, because of large bandwidths and narrow time widths.large bandwidths and narrow time widths.
Spread spectrum communications (3G) offer Spread spectrum communications (3G) offer high bandwidth compared to 1G and 2G high bandwidth compared to 1G and 2G systems.systems.
Resistance to multi-path fading, because of Resistance to multi-path fading, because of large bandwidths and narrow time widths.large bandwidths and narrow time widths.
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PN Sequences PN Sequences PN Sequences PN Sequences Pseudo Noise sequence is a binary sequence Pseudo Noise sequence is a binary sequence
of 1s and -1s of 1s and -1s PN sequences are generated by using PN sequences are generated by using
sequential logic circuitssequential logic circuits Very low cross correlation exists between any Very low cross correlation exists between any
two PN sequencestwo PN sequences High cross correlation exists between High cross correlation exists between
identical PN sequencesidentical PN sequences
Pseudo Noise sequence is a binary sequence Pseudo Noise sequence is a binary sequence of 1s and -1s of 1s and -1s
PN sequences are generated by using PN sequences are generated by using sequential logic circuitssequential logic circuits
Very low cross correlation exists between any Very low cross correlation exists between any two PN sequencestwo PN sequences
High cross correlation exists between High cross correlation exists between identical PN sequencesidentical PN sequences
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Frequency Hopped Spread spectrum (FHSS)Frequency Hopped Spread spectrum (FHSS)Frequency Hopped Spread spectrum (FHSS)Frequency Hopped Spread spectrum (FHSS)
A frequency hopping signal periodically A frequency hopping signal periodically changes the carrier frequency by using PN changes the carrier frequency by using PN controlcontrol
The set of possible carrier frequencies is called The set of possible carrier frequencies is called a a hopsethopset
Bandwidth of channel used in hopset Bandwidth of channel used in hopset Instantaneous bandwidth BInstantaneous bandwidth B
Bandwidth of spectrum over which the hopping Bandwidth of spectrum over which the hopping occurs occurs total hopping bandwidth W total hopping bandwidth Wssss
A frequency hopping signal periodically A frequency hopping signal periodically changes the carrier frequency by using PN changes the carrier frequency by using PN controlcontrol
The set of possible carrier frequencies is called The set of possible carrier frequencies is called a a hopsethopset
Bandwidth of channel used in hopset Bandwidth of channel used in hopset Instantaneous bandwidth BInstantaneous bandwidth B
Bandwidth of spectrum over which the hopping Bandwidth of spectrum over which the hopping occurs occurs total hopping bandwidth W total hopping bandwidth Wssss
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Methodology of FHSSMethodology of FHSSMethodology of FHSSMethodology of FHSS Time duration between hops Time duration between hops hopping period hopping period
TTss
Data is sent by hopping the transmitter carrier Data is sent by hopping the transmitter carrier over the hopset generated by PN codesover the hopset generated by PN codes
Small bursts of data are sent before T/R hops Small bursts of data are sent before T/R hops againagain
Hit => Two users using the same frequency Hit => Two users using the same frequency band at the same timeband at the same time
Time duration between hops Time duration between hops hopping period hopping period TTss
Data is sent by hopping the transmitter carrier Data is sent by hopping the transmitter carrier over the hopset generated by PN codesover the hopset generated by PN codes
Small bursts of data are sent before T/R hops Small bursts of data are sent before T/R hops againagain
Hit => Two users using the same frequency Hit => Two users using the same frequency band at the same timeband at the same time
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Frequency Hopping ModulatorFrequency Hopping ModulatorFrequency Hopping ModulatorFrequency Hopping Modulator
ModulatorModulator
PN CodePN CodeGeneratorGenerator
Frequency Frequency SynchronizerSynchronizer
Code BlockCode Block
DATADATA
Frequency Frequency Hopping Hopping
SignalSignal
OscillatorOscillator
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Frequency hopping demodulatorFrequency hopping demodulatorFrequency hopping demodulatorFrequency hopping demodulator
Wideband Wideband FilterFilter
PN Code PN Code GeneratorGenerator
Frequency Frequency SynthesizerSynthesizer
SynchronizationSynchronizationSystemSystem
BP FilterBP Filter DemodulationDemodulationDATADATA
Frequency Frequency Hopping Hopping
SignalSignal
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Properties of FHSSProperties of FHSSProperties of FHSSProperties of FHSS Fast frequency hoppingFast frequency hopping
More than one frequency hop during each More than one frequency hop during each transmitted symbol transmitted symbol
-> Hopping rate ≥ symbol rate-> Hopping rate ≥ symbol rate
Slow frequency hoppingSlow frequency hopping
Hopping rate < symbol rateHopping rate < symbol rate
Fast frequency hoppingFast frequency hopping
More than one frequency hop during each More than one frequency hop during each transmitted symbol transmitted symbol
-> Hopping rate ≥ symbol rate-> Hopping rate ≥ symbol rate
Slow frequency hoppingSlow frequency hopping
Hopping rate < symbol rateHopping rate < symbol rate
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Parameters of FH-SSParameters of FH-SSParameters of FH-SSParameters of FH-SS Probability of error for BPSK Spread SpectrumProbability of error for BPSK Spread Spectrum
PPee = 0.5 x e = 0.5 x e -E-Ebb/ 2N/ 2N00 x (1 – p x (1 – ph h ) + 0.5 p) + 0.5 phh
pphh = probability of hit = 1 – (1 – 1/M) = probability of hit = 1 – (1 – 1/M)K-1K-1
M = number of hopping channelsM = number of hopping channels
K = Total number of usersK = Total number of users Processing gain (PG) = WProcessing gain (PG) = Wssss / B / B
Probability of error for BPSK Spread SpectrumProbability of error for BPSK Spread Spectrum
PPee = 0.5 x e = 0.5 x e -E-Ebb/ 2N/ 2N00 x (1 – p x (1 – ph h ) + 0.5 p) + 0.5 phh
pphh = probability of hit = 1 – (1 – 1/M) = probability of hit = 1 – (1 – 1/M)K-1K-1
M = number of hopping channelsM = number of hopping channels
K = Total number of usersK = Total number of users Processing gain (PG) = WProcessing gain (PG) = Wssss / B / B
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Direct Sequence Spread Spectrum (DSSS)Direct Sequence Spread Spectrum (DSSS)Direct Sequence Spread Spectrum (DSSS)Direct Sequence Spread Spectrum (DSSS)
codecode
timetime
frequencyfrequency
CC11
CC22
CCNN
4646
Properties of DSSS signalProperties of DSSS signalProperties of DSSS signalProperties of DSSS signal Message signal is a time sequence of non-Message signal is a time sequence of non-
overlapping pulses of duration T, each of which overlapping pulses of duration T, each of which has an amplitude (+/-) 1.has an amplitude (+/-) 1.
The PN waveform consists of N pulses or chips The PN waveform consists of N pulses or chips for message symbol period T.for message symbol period T.
NTNTC C = T= T
where Twhere TCC is the chip period. is the chip period.
Message signal is a time sequence of non-Message signal is a time sequence of non-overlapping pulses of duration T, each of which overlapping pulses of duration T, each of which has an amplitude (+/-) 1.has an amplitude (+/-) 1.
The PN waveform consists of N pulses or chips The PN waveform consists of N pulses or chips for message symbol period T.for message symbol period T.
NTNTC C = T= T
where Twhere TCC is the chip period. is the chip period.
4747
ExampleExample:: ExampleExample:: N=4N=4
PN Wave for N =4
11
-1-1
-1-1
11
4848
DSSS TransmitterDSSS TransmitterDSSS TransmitterDSSS Transmitter
1m (t)
1S (t)
11
kk
1PN (t)
KPN (t)
c 1cos(2 f t )
r(t)
c kcos(2 f t )
mk(t)
4949
Principles of transmitter operationPrinciples of transmitter operationPrinciples of transmitter operationPrinciples of transmitter operation The narrowband message signal mThe narrowband message signal m ii(t) is (t) is
multiplied by a pseudo noise code multiplied by a pseudo noise code sequence that has a chip rate >> data rate sequence that has a chip rate >> data rate of message.of message.
All users use the same carrier frequency All users use the same carrier frequency and may transmit simultaneously. The kth and may transmit simultaneously. The kth transmitted signal is given by:transmitted signal is given by:
The narrowband message signal mThe narrowband message signal m ii(t) is (t) is multiplied by a pseudo noise code multiplied by a pseudo noise code sequence that has a chip rate >> data rate sequence that has a chip rate >> data rate of message.of message.
All users use the same carrier frequency All users use the same carrier frequency and may transmit simultaneously. The kth and may transmit simultaneously. The kth transmitted signal is given by:transmitted signal is given by:
k s s k k c kS (t) (2E / T )1/ 2m (t)p (t)cos(2 f t )
5050
CDMA ReceiverCDMA ReceiverCDMA ReceiverCDMA Receiver
c kcos(2 f t )
r(t)km (t)
(.)dt(.)dt
kiZ (t)kiZ (t)
KPN (t)
5151
Principles of receiver operationPrinciples of receiver operationPrinciples of receiver operationPrinciples of receiver operation At the receiver, the received signal is At the receiver, the received signal is
correlated with the appropriate PN sequencecorrelated with the appropriate PN sequence
The multiplied signal will be pThe multiplied signal will be p22(t) = 1 for the (t) = 1 for the correct signal and can be demodulated to correct signal and can be demodulated to yield the message signal myield the message signal m ii(t)(t)
At the receiver, the received signal is At the receiver, the received signal is correlated with the appropriate PN sequencecorrelated with the appropriate PN sequence
The multiplied signal will be pThe multiplied signal will be p22(t) = 1 for the (t) = 1 for the correct signal and can be demodulated to correct signal and can be demodulated to yield the message signal myield the message signal m ii(t)(t)
1
1
iT1i 1 1 c 1 1
(i 1)T
Z (t) r(t)p (t )cos[2 f (t ) ]dt
5252
Parameters of DSSSParameters of DSSSParameters of DSSSParameters of DSSS Probability of bit error (BER) Probability of bit error (BER)
PPee = Q {1/ [(K –1)/3N + (N = Q {1/ [(K –1)/3N + (N00/2E/2Ebb)])]1/21/2}}
KK = Number of users= Number of users
NN = Number of chips/ symbol= Number of chips/ symbol
When EWhen Ebb/N/Noo
PPee = Q{[3N/(K-1)] = Q{[3N/(K-1)]1/21/2 } }
Probability of bit error (BER) Probability of bit error (BER)
PPee = Q {1/ [(K –1)/3N + (N = Q {1/ [(K –1)/3N + (N00/2E/2Ebb)])]1/21/2}}
KK = Number of users= Number of users
NN = Number of chips/ symbol= Number of chips/ symbol
When EWhen Ebb/N/Noo
PPee = Q{[3N/(K-1)] = Q{[3N/(K-1)]1/21/2 } }
5353
Important Advantages of CDMAImportant Advantages of CDMAImportant Advantages of CDMAImportant Advantages of CDMA
Many users of CDMA use the same frequencyMany users of CDMA use the same frequency Multipath fading may be substantially reduced Multipath fading may be substantially reduced
because of large signal bandwidth.because of large signal bandwidth. There is no absolute limit on the number of There is no absolute limit on the number of
users in CDMA users in CDMA System performance gradually degrades for all System performance gradually degrades for all
users as the number of users is increased.users as the number of users is increased.
Many users of CDMA use the same frequencyMany users of CDMA use the same frequency Multipath fading may be substantially reduced Multipath fading may be substantially reduced
because of large signal bandwidth.because of large signal bandwidth. There is no absolute limit on the number of There is no absolute limit on the number of
users in CDMA users in CDMA System performance gradually degrades for all System performance gradually degrades for all
users as the number of users is increased.users as the number of users is increased.
5454
Drawbacks of CDMADrawbacks of CDMADrawbacks of CDMADrawbacks of CDMA
Self-jamming is a problem in a CDMA system. Self-jamming is a problem in a CDMA system. Self-jamming occurs because the PN Self-jamming occurs because the PN sequences are not exactly orthogonal.sequences are not exactly orthogonal.
The near- far problem occurs at a CDMA The near- far problem occurs at a CDMA receiver if an undesired user has high detected receiver if an undesired user has high detected power as compared to the desired user.power as compared to the desired user.
Self-jamming is a problem in a CDMA system. Self-jamming is a problem in a CDMA system. Self-jamming occurs because the PN Self-jamming occurs because the PN sequences are not exactly orthogonal.sequences are not exactly orthogonal.
The near- far problem occurs at a CDMA The near- far problem occurs at a CDMA receiver if an undesired user has high detected receiver if an undesired user has high detected power as compared to the desired user.power as compared to the desired user.
5555
Modulation performance in fading channelsModulation performance in fading channelsModulation performance in fading channelsModulation performance in fading channels
s(t) r(t) s(t) r(t)
r(t) = r(t) = (t) e(t) e-j-j(t)(t) s(t) + n(t) s(t) + n(t)
(t) = gain of the channel(t) = gain of the channel
(t) = phase shift of the channel(t) = phase shift of the channel
n(t) = additive Gaussian noisen(t) = additive Gaussian noise
s(t) r(t) s(t) r(t)
r(t) = r(t) = (t) e(t) e-j-j(t)(t) s(t) + n(t) s(t) + n(t)
(t) = gain of the channel(t) = gain of the channel
(t) = phase shift of the channel(t) = phase shift of the channel
n(t) = additive Gaussian noisen(t) = additive Gaussian noise
Fading ChannelFading Channel
5656
Probability of error in fading channelsProbability of error in fading channelsProbability of error in fading channelsProbability of error in fading channels
Probability of error (BER)Probability of error (BER)
= BER for parent modulation (BPSK, FSK)= BER for parent modulation (BPSK, FSK)
= (E= (EBB / N / N00) ) 22
Probability of error (BER)Probability of error (BER)
= BER for parent modulation (BPSK, FSK)= BER for parent modulation (BPSK, FSK)
= (E= (EBB / N / N00) ) 22
e e0
P P (X)p(X)dX
eP (X)eP (X)
p(X) = pdf of X due to fading channel p(X) = pdf of X due to fading channel
= = (1/ )exp( x / ),x 0
5757
PPee (X) and P (X) and Pe e for different systems for different systemsPPee (X) and P (X) and Pe e for different systems for different systems
Coherent binary PSKCoherent binary PSK
Coherent binary FSKCoherent binary FSK
Coherent binary PSKCoherent binary PSK
Coherent binary FSKCoherent binary FSK
1/ 2e B 0P (X) Q[(2E /N ) ]
eP 0.5[1 /(1 )]
1/ 2e B 0P (X) Q[(E /N ) ]
eP 0.5[1 /(2 )]
5858
Differential Binary PSKDifferential Binary PSK
Non-coherent orthogonal binary FSKNon-coherent orthogonal binary FSK
Differential Binary PSKDifferential Binary PSK
Non-coherent orthogonal binary FSKNon-coherent orthogonal binary FSK
e B 0P (X) 0.5exp[( E /N )]
eP [0.5 /(1 )]
e B 0P (X) 0.5exp[( E / 2N )]
eP [0.5 /(2 )]
5959
Coherent GMSK Coherent GMSK
=0.68, BT = 0.25, =0.68, BT = 0.25, = 0.68 = 0.68
=0.85, BT = =0.85, BT = , , = 0.85 = 0.85
BT = Bandwidth – bit duration product for GMSKBT = Bandwidth – bit duration product for GMSK
Coherent GMSK Coherent GMSK
=0.68, BT = 0.25, =0.68, BT = 0.25, = 0.68 = 0.68
=0.85, BT = =0.85, BT = , , = 0.85 = 0.85
BT = Bandwidth – bit duration product for GMSKBT = Bandwidth – bit duration product for GMSK
e B
1/ 2e
P (X) Q[(2 E )]
P 0.5{1 [ /( 1)] 1/ 4
e B
1/ 2e
P (X) Q[(2 E )]
P 0.5{1 [ /( 1)] 1/ 4